Optical information readers that aim at optically reading information symbols, such as barcodes, QR codes, and the like, attached to goods and documents have been well known.
Such optical information readers are each made up of a handheld body case provided at its one end portion with a reading window; a photodetector, such as a CCD (Charge-Coupled Device) area sensor; an imaging unit with an imaging lens; and a light illuminating device, such as a LED (Light Emitting Diode). The photodetector, the imaging unit, and the light illuminating device are installed in the body case.
In this structure of the optical information reader, when a user wants to read an information symbol attached to a goods, the user for example locates the optical information reader so that the reading window is opposite to the goods and away therefrom at an arbitrary distance.
While the optical information reader is arranged in such a state, the light illuminating device works to supply illumination light through the reading window toward an information symbol to be irradiated thereto. Light reflected from the information symbol based on the irradiated illumination light enters through the reading window into the imaging unit.
The light entered into the imaging unit is focused on the photodetector by the imaging lens to be imaged thereon, so that an image corresponding to the information symbol is picked up by the photodetector. Based on the contrast pattern between light (white) pixels and dark (black) pixels in the picked-up image, information stored in the information symbol can be decoded.
When such an optical information reader is used to read an information symbol printed on a calendered paper, the illumination light irradiated on the information symbol on the calendered paper may be specularly reflected depending on an incident angle of the illumination light with respect to the calendered paper. The incident angle of the illumination light with respect to a surface of a target for reading means an angle of the illumination light with respect to the normal of the surface. The incident angle will also be referred to as a reading angle hereinafter.
This specular reflection (mirror reflection) may cause at least part of the information symbol to be picked up as light pixels (white pixels) independently of the light and dark information stored in the corresponding at least part of the information symbol. This may cause improper reading of the information stored in the information symbol
Particularly, in reading an information symbol directly marked on a metal surface of goods in direct marking, such specular reflection may likely appear, making it difficult to properly read the information stored in the information symbol.
In such cases where the information stored in an information symbol is improperly read by an optical information reader, the optical information reader is designed to determine that a target for reading is abnormal. For this reason, a user periodically tries to:
rearrange the optical information reader in front of the target information symbol while changing the position of the optical information reader relative to the target information symbol and/or the reading angle of the optical information reader with respect thereto; and
perform the image reading operations every rearranging.
This may deteriorate the efficiency of reading the information stored in the information symbol.
In order to reduce the influence of specular reflection, some conventional optical information readers have been prepared.
As a first example, U.S. Pat. Publication No. 6,394,349 corresponding to Japanese Unexamined Patent Publication No. H11-120284 discloses an optical information reader.
The optical information reader as the first example is designed to:
irradiate a target information code by using one combination of a plurality of illumination light beams whose irradiating directions with respect to the information code are different from each other;
read a first image of the information code using light reflected by the information code based on the one combination of the plurality of illumination light beams being irradiated on the information code;
determine whether a specular reflection region exists in the first image;
irradiate the target information code by using another combination of the plurality of illumination light beams when it is determined that a specular reflection region exists in the first image;
read a second image of the information code using light reflected by the information code based on another combination of the plurality of illumination light beams being irradiated on the information code; and
combine the first image and the second image, thus reading the combined image.
As a second example, Japanese Unexamined Patent Publication No. S59-41088 discloses an optical information reader designed to mechanically change the incident angle of an illumination light with respect to an information code when optically detecting the occurrence of specular reflection.
However, in the optical information reader of the second example, the structure of the mechanical change of the incident angle may increase in complexity.
As a third example, Japanese Unexamined Patent Publication No. H02-98789 discloses an image reader with first and second image pickup devices. The first and second image pickup devices are disposed opposing to a calendered surface of a target plate on which a character string has been printed such that whose optical axes are orthogonal thereto and parallel to each other. When the first and second image pickup devices work to respectively pick up corresponding first and second images of the target, the first and second images are written into corresponding first and second memories, respectively.
Thereafter, an image superimposing circuit of the image reader is designed to carry out image superimposing tasks to:
associate all addresses of the first memory with those of the second memory such that a part of the first image of the same part of the target corresponds to that of the second image thereof;
compare the light intensity data of each pixel of the first image data with that of a corresponding one pixel of the second image data;
select the light intensity data of some pixels of the first image data to write it into a superimposing image memory when it is determined that the light intensity of some pixels of the first image data is lower than that of corresponding some pixels of the second image data based on the comparison result; and
select the light intensity data of the remaining pixels of the second image data to write it into the superimposing image memory when it is determined that the light intensity of the remaining pixels of the first image data is lower than that of the corresponding remaining pixels of the second image data based on the comparison result.
However, in the image reader of the third example, because the image superimposing task is carried out pixel by pixel of each of the first and second images of the target, it may increase in complexity.
In addition, in direct marking, an information symbol is directly printed on a surface of a target part. For this reason, there are many information symbols to be used for directly marking, whose unit section of information contained therein, such as a cell in QR codes, is smaller in size than that of information contained in another information symbol to be used except for the directly marking.
In order to read an information symbol consisting of a plurality of unit sections of information having a comparatively small size, the angle of view of an imaging unit of an optical information reader is set to be narrow. This allows the number of pixels of a photodetector to be allocated to each unit section of information of the information symbol to increase. Note that, in this specification, the angle of view of an imaging unit means the angle of visible field of view measured from the center of an imaging lens. In other words, the angle of view of an imaging unit means the angle of a field of view of a photodetector to be imaged thereby.
However, the narrower the angle of view of an imaging lens is, the narrower the field of view of a photodetector is. For this reason, an information reader with an imaging unit having a narrow angle of view can pick up an image of an information symbol consisting of a plurality of comparatively small-sized unit sections of information. However, it may be difficult for an information reader with an imaging unit having a narrow angle of view to pick up an image of an information symbol consisting of a plurality of comparatively large-sized unit sections of information. This is because the information symbol consisting of a plurality of comparatively large-sized unit sections of information may extend off the filed of view of the photodetector.